Iris, the Greek goddess of the rainbow, carried the first messages from the gods to man; 3,000 years later, the flow of communication is to be reversed. There are plans afoot, as we learned this week, to harness our irises, those pretty rings of multicoloured muscle in our eyes, to reveal our identities to the Olympians of Homeland Security.

We'll each need to earn notoriety first: the FBI's data-sharing proposals, involving an entire suite of biometric data, are directed at catching major criminals and terrorists. The name the Feds gave this project, however, suggests that someone, somewhere, is looking to the future: "server in the sky". This is either a tip of the hat to 80s rock band Doctor and the Medics' only hit or, more likely, a grotesque piece of security-state triumphalism.

Mind you, we are all more than likely to offer up our eyes over the next couple of years to any institution that cares to stare into them. Iris scanning is set to replace the passport and credit card as the preferred method of proving identity. Who wouldn't want to pass through Heathrow in a blink, after all?

But there is something unpleasant about the idea of having one's eyes scanned, and this is not altogether the fault of the film Minority Report's stolen eyeballs scene. It is more to do with our intuition that the eyes are windows on the soul. The human eye is built to be noticed. Simply opening the eyes wider can, with other facial movements, express everything from shock to arousal to doubt. Simple gaze direction conveys emotional meaning. The lateral rectus eye muscle is labelled "amatoris" in early anatomies because lovers use it to direct their flirtatious glances.

Eyes reveal our inner state. It is impossible to control our rate of blinking for any length of time, or the way our pupils wax and wane. When aroused, we blink more often, and our irises dilate. Our eyes, with their bright whites, colourful irises, responsive pupils, brows and lashes, have evolved to communicate and carry meaning.

Nonetheless, given the amount of information they carry, eyes are surprisingly hard to read. We don't count each other's blinks, and we don't press our faces up against each other to study the changes in each other's irises. Of course, we don't have to: we have language - which lets us lie in a way the eyes don't. But liars are easy to spot - aren't they?

Humans have been pack animals for most of their history. When survival depended on cooperation there was little advantage to be had from blatant lying. In a tightknit community, a pathological liar stands to lose too much if they are caught out. Now, things are different. A 65-year-old, Jean Hutchinson, was sent to jail for five years this week. Why? From her secret operations room, accessed through a wardrobe, she had managed to impersonate 76 different people well enough to defraud the British state of £2.4m.

Technology confers anonymity on people far more effectively than it establishes identity. The biometric security market emerged in the US following the passing of two laws. Neither had anything to do with security, the war on terror or other bugaboos. One was the health insurance act of 1996, which made healthcare firms protect their clients' records more carefully; the other, known as Sarbanes-Oxley, was meant to reduce the fiddling of financial records after the collapse of Enron.

The war on terror is a branding exercise. The war on fraud is real. The technology has a long way to go before machines are invented that can scan our eyes for the secrets of our hearts. Still, this is the path we are on. As our machines learn more about us, we are increasingly learning how to hide behind our machines.

· Simon Ings's The Eye: A Natural History is published next week by Bloomsbury simonings@gmail.com